Due to the practical adoption of ultrashort pulsed laser technology in recent years, emission techniques and detection techniques for pulsed, coherent, far-infrared (particularly in the terahertz region) electromagnetic waves have progressed rapidly. Accordingly, time-domain pulsed spectroscopy using these pulsed far-infrared electromagnetic waves has become possible, and the pioneering development of practical time-domain pulsed spectroscopy apparatuses has progressed in Japan too.
Time-domain pulsed spectroscopy is a spectroscopy method in which, by measuring the time-dependent electric field intensity of the pulsed electromagnetic field and by Fourier transforming this time-dependent data (time-series data), the electric field intensity and phase of individual frequency components forming this pulse are obtained. One feature of this spectroscopy method is that the measurement wavelength range is the boundary region between light and electromagnetic waves, which is difficult to achieve with conventional measurement. Therefore, this spectroscopy method is expected to elucidate the properties of novel materials and new phenomena. Furthermore, only the electric field intensity of an electromagnetic wave can be obtained with conventional spectroscopy methods; however, this time-domain pulsed spectroscopy method has the unique feature that, by directly measuring temporal changes in the electric field intensity of electromagnetic waves, it can obtain not only the electric field intensity (amplitude) of the electromagnetic waves, but also the phase thereof. Therefore, it is possible to obtain a phase-shift spectrum by comparison with a case where there is no sample. Because the phase shift is proportional to the wave vector, it is possible to determine the dispersion relation in the sample using this spectroscopy method, and it is also possible to determine the dielectric constant of a dielectric from this dispersion relation (see Japanese Unexamined Patent Application Publication No. 2002-277394).
FIG. 1 shows one example of a conventional time-domain pulsed spectroscopy apparatus.
Reference numeral 1 is a light source for emitting femtosecond laser. Femtosecond laser light L1 emitted from the light source 1 is split at a beam splitter (splitting unit) 2. One of the femtosecond lasers is radiated onto a pulsed-light emitting unit 5 as excitation pulsed laser light (pump pulsed light) L2. At this time, after being modulated by an optical chopper 3, the excitation pulsed laser light L2 is focused by an objective lens 4. This pulsed-light emitting unit 5 is, for example, a photoconductive element in which an electric current flows momentarily when the excitation pulsed laser light L2 is radiated, and emits a far-infrared electromagnetic pulse. This far-infrared electromagnetic pulse is guided by parabolic mirrors and is irradiated onto a measurement sample 8. Reflected or transmitted pulsed electromagnetic waves (in this example, transmitted pulsed electromagnetic waves) from the sample 8 are guided to a detector 12 by parabolic mirrors 9 and 10.
The other laser light split at the beam splitter 2 is guided to the detector 12 as detection pulsed laser light (sampling pulsed light) L3. This detector 12, which is also, for example, a photoconductive element, becomes conductive only for the instant when the detection pulsed laser light L3 is irradiated; therefore, it is possible to detect the electric field intensity of the reflected or transmitted pulsed electromagnetic waves from the sample 8, arriving at that instant, as an electrical current. A wave form signal of the electric field intensity of the reflected or transmitted pulsed electromagnetic waves from the sample 8 can be obtained by applying a delay time at predetermined time intervals to the detection pulsed laser light L3 with respect to the excitation pulsed laser light L2 using an optical delay unit 13 (or 14). In this example, in addition to the optical delay unit 13 (or 14) for wave form signal measurement, an optical delay unit 14 (or 13) for adjusting the temporal origin is also provided.
Each item of time-resolved data of the electric field intensity of the reflected or transmitted electromagnetic waves from the sample 8 is processed by a signal processing unit. More specifically, the data is transferred to a computer 17 via a lock-in amplifier 16 and is then stored as time-series data, and amplitude and phase spectra of the electric field intensity of the reflected or transmitted electromagnetic waves from the sample 8 are obtained by applying Fourier transform processing to one sequence of time-series data in the computer 17 to transform it into vibration-frequency (frequency) space.
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[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-121355.
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[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2003-75251.
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 2003-14620.
[Patent Document 6] Japanese Unexamined Patent Application Publication No. 2002-277393.
[Patent Document 7] Japanese Unexamined Patent Application Publication No. 2002-277394.
[Patent Document 8] Japanese Unexamined Patent Application Publication No. 2002-257629.
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